1. Field of the Invention
The present invention relates to the detection and location of underwater objects located in the vicinity of the surface of the water by means of a frontal active sonar mounted on a surface vessel.
2. Description of the Prior Art
The sonars currently on the market are little suited to the detection of sub-surface underwater objects for different reasons. Their directivity in elevation is generally not optimized for observing the surface. Furthermore, the surface reverberation, especially when the sea is rough, creates a stray signal that masks the echo received from the object. Finally, these sonars possess no device for measuring the submersion of the target.
However, there exists a sonar system, designed chiefly to be mounted on a towed "fish", in which several channels in elevation, covering the entire height of water, are formed at reception. Since this sonar is chiefly designed for detection, its working frequency is low and, consequently, its directivity is not fine. The distance measurement or monopulse technique performed on both signals of two adjacent elevation channels is used to localize an object in elevation. This processing operation is done for each channel in relative bearing.
The amplitude distance measurement or amplitude monopulse technique consists in measuring the difference and the sum of the amplitudes of two signals received simultaneously by two adjacent channels, the patterns of directivity of which are identical, with the same phase center and pointed in two different directions O.sub.1 and O.sub.2. The difference/sum ratio then makes it possible to obtain the direction of the echo with respect to the mean direction (O.sub.1 +O.sub.2)/2. The direction of the echo can also be obtained by phase distance measurement in forming two channels with two half-antennas so as to have offset phase centers.